The present invention relates to a method and apparatus for processing video signals such as facsimile or the like signals and reconstructing the original through a conventional recording system.
In digital systems where the video signal is converted into digital samples, the picture quality of the digitized image tends to be affected considerably by a moire pattern resulting from interference beats between a periodic pattern present in the original and a patteren generated as a result of the quantization.
The recent rapid growth in facsimile communications have created various demands not only for transmitting documents, but also a wide variety of pictorial images having inherent periodic patterns. In particular, if the original has a fine periodic structures such as textures, the periodic patterns interfere with the periodic intervals with which the original is scanned, producing a moire pattern in a reconstructed image. This imposes limitations on the capability of a facsimile communication system. To overcome this difficulty various attempts have been made to increase the scan density and to provide variable density scan with an attendant increase in cost. The following is a description of the disadvantages associated with the facsimile transmission of screen dot photographs currently employed by newspaper companies.
The screen dot photograph is made up of a multitude of black dots and their arrangement defines the image. The current method involves segmenting the dot into a matrix of squares and quantizing the black area of the square and assigning to it a specific quantum number. If the assigned quantum number is smaller than a threshold value of 50, for example, the square is taken as white and if it exceeds a threshold 51 it is treated as black. The original dot is thus represented by a plurality of squares or picture elements and the number of such elements depends on the relative size and position of each square in the original dot.
Because of the quantization the reconstructed image is an approximation of the original and differs significantly in some squares where the number of dots contained therein is near the threshold values. The difference in dot number between the original and the reconstructed image repreprents a difference in average image density.
Moire patterns would appear in a reconstructed image when it contains a series of squares that vary periodically in average density even if the original dots have a uniform distribution. One approach to making the moire pattern less noticeable to the eyes would be to achieve uniformity in average image density if the original dots have a uniform distribution.
Eliminating the moire pattern could be achieved by scanning the original with a tiny spot much smaller than the dot size since it minimizes errors and thus reduces point-to-point variation in image density. However, the amount of digitized samples increases in proportion to the square value of the number of scan lines and results various disadvantages including an increase in data processing time, system complexity, low transmission line efficiency and a large memory capacity for a system of the type where a screen memory is employed for processing. The scan density has therefore been chosen to minimize the effect of moire patterns so that such patterns are less noticeable to the eyes. For example, the square size is chosen to be a submultiple of the dot size and squares are aligned parallel with dots to minimize the deviation of the reconstructed dot shape from the original. However, this approach proves unsatisfactory because the dots are not necessarily aligned in desired directions and their size and shape vary from print to print.
On the other hand, demands have arisen to transmit halftone images through binary coded signals. A typical example of this type of transmission involves the use of dither method. However, the dither method is not satisfactory for discretely valued images such as screen dot photographs, documents and line drawings, while it is satisfactory to halftone images.